A variety of methods have been proposed for the remediation of soil containing organic contaminants. Many of the proposed methods involve removal and subsequent incineration of soil with the attendant difficulties of treatment and/or disposal of off-gases. A major detriment to such processes, however, is the cost of evacuating and transporting the soil which can result in a total cost approaching 500 U.S. dollars per ton of soil.
To avoid at least a portion of these costs, several types of in-situ processes have been proposed including vitrification of the soil by electrode heating, steam or hot air heating of the soil through an auger system or through stationary pipes, and radio-frequency or electrical heating of the soil by means of a surface heater.
Brouns et al, U.S. Pat. No. 4,376,598, disclose a vitrification process in which the soil is heated to approximately 1500.degree. C. At or about this temperature the soil forms a glass-like mass which traps the contaminants therein. This process, in reality, is a stabilization process rather than a decontamination process since the soil undergoing treatment has lost its physical and chemical identity. Both an auger system for injecting steam or hot air and a process for steam injection through stationary pipes have been practiced commercially. These methods have a limited use, primarily in the decontamination of soil containing small areas of deep contamination such as localized spill or leakages at service stations. These methods are not as useful when applied to large areas of contaminated soil.
Bridges et al, U.S. Pat. No. 4,670,634, disclose an in-situ thermal process where the thermal energy is supplied by radio-frequency heating. This process is particularly applicable to water-containing soils where the steam generated in the soil serves to strip the organic contaminants from the soil. A somewhat related process is disclosed by assignee's U.S. patent application Ser. No. 427,418, filed Oct. 27, 1989, now U.S. Pat. No. 4,984,594 wherein the thermal energy is supplied by a relatively flat heater deployed at the surface of the soil. In this latter process, a lowered pressure is applied at the surface of the soil to remove vapors generated within the soil. This method is somewhat inefficient since significant vapor flow takes place only at the surface of the soil or within a "blanket" placed on the surface of the soil which is more permeable to vapor flow than the soil it covers. It would be of advantage to provide a more effective method of collecting and removing from heated soil the vapors formed when soil contaminated by organic contaminants is heated.
U.S. Pat. No. 4,842,448 issued to Robert M. Koerner et al on Jun. 27, 1989 discloses a method and apparatus for in-situ removal of contaminants from soil comprising a barrier having a permeable inner layer and an impermeable outer layer overlying the contaminated soil and a vacuum system for reducing pressure under the barrier and withdrawing contaminants from the contaminated soil.
In Assignee's co-pending application Ser. No. 427,427 filed Oct. 27, 1989, now abandoned in favor of continuation application Ser. No. 833,569, filed Feb. 7, 1992, an in-situ method is disclosed for remediation and decontamination of surface and near-surface soils by electrically heating the soil through electrodes operated at power line frequencies of about 60 Hz. The electrodes are implanted substantially vertically in the soil in a line pattern which allows substantially uniform electrical heating in the region between rows of electrodes. The depth of electrode insertion is substantially equal to the depth of the contamination, but could be deeper or shallower. The process is particularly applicable to soils contaminated at depths of up to about 30 meters. The electrodes are hollow and perforated below the surface to allow application of a vacuum to the soil through the electrodes. The electrodes are also equipped with provision for electrical connection and vacuum line connection, and also with the capability to be sealed to a barrier that is impermeable to gases, such as a flexible sheet.
U.S. Pat. No. 4,435,292 discloses a portable system which can be installed at an area where a contaminating spill has occurred. After the area of the contamination has been determined, perforated pipes are inserted into the ground. Some of the wells are pressurized and others are evacuated simultaneously so as to increase the transfer of a flushing fluid through the soil thereby accelerating the decontamination process and preventing migration of the contaminant into other areas. Since the system is a closed system, the contaminated liquid taken off in the evacuation side of the circulating system is bypassed in whole or in part to a portable processing plant wherein the contaminants are removed.
And, in Assignee's co-pending application Ser. No. 559,771 filed Jul. 30, 1990, now U.S. Pat. No. 5,076,727, moist warm air from a vapor treatment system is injected into wells which are screened (perforated) only at the contaminated depth forcing vapor flow only through the contaminated region. Between the injection wells is an extraction well which is also screened only at the contaminated depth. A vacuum is drawn on the extraction well through the contaminated soil, thereby entraining some of the contaminants. The contaminated, flushing vapor is then treated and recycled. A microwave/radio frequency (MW/RF) heating system heats the earth's surface and the contaminated soil, thereby enhancing volatilization of the contaminants and their removal via the vapor flushing system. By screening the wells only through the contaminated zone and maintaining the contaminated soil zone in a moist state, the entire energy of the system is focussed on the contaminated region.
In Assignee's copending application Ser. No. 627,479, filed Dec. 14, 1990 an in-situ method for removal of contaminants from soil imposes a vacuum on the soil through perforated heater wells that are positioned in the soil. The heater wells heat the soil to elevated temperatures by thermal conduction. The heater wells are permeable to vapors which emanate from the soil when heated and which are drawn towards the heater wells by the imposed vacuum. An impermeable flexible sheeting on the soil surface reduces the amount of air that is being pulled into the heater well from the atmosphere. A thermal insulator covers the soil surface and reduces heat losses from the soil surface. The heater wells are connected to a vacuum manifold for collection of vapors. A heat front moves away from the heater wells through the soil by thermal conduction, and the superposition of heat from a plurality of heater wells results in a more uniform temperature rise throughout the well pattern. Soil contaminants are removed by vaporization, in-situ thermal decomposition, oxidation, combustion, and by steam distillation. The heater wells and the nearby soil are extremely hot and many contaminants drawn into the wells will decompose with a residence time on the order of seconds. The heater well can also be packed with a catlyst that accelerates high temperature decomposition into simpler molecules. Water vapor and remaining contaminants may be incinerated in line or may be collected in a cold trap upstream from the vacuum pump.
In Assignee's co-pending application Ser. No. 705,712, filed concurrent herewith an in-situ thermal desorption system also utilizes perforated or slotted pipe buried in the soil below the depth of contamination in the soil. The surface of the soil is covered with a layer of permeable insulation (to conserve heat and to provide a gas migration path on top of the soil) and a layer of impermeable material above the insulation. A vapor recovery/treatment system located at the surface consists of a method of inducing a vacuum between the impermeable layer and the soil surface (e.g., a vacuum pump or an induced draft fan) and a treatment system for the contaminated vapor (e.g., a cold trap, carbon adsorption, or incineration). Fuel and compressed air are fed to a pressurized combustion chamber and combusted, the combustion products flow into the buried pipe and are distributed through the contaminated soil. Heat from the pressurized combustion products causes the organic contaminants within the soil to vaporize, pyrolyze, decompose, or react with oxygen. Contaminants and their by-products are swept away by the combustion products into the vapor recovery/treatment system.